Blood flow measuring apparatus

ABSTRACT

A blood flow measuring apparatus has a measuring electrode and a counter-electrode adapted to be brought into contact with the blood of a patient. A measuring unit measures a voltage or a current between the measuring electrode and the counter-electrode and determines the flow of blood therefrom. The measuring electrode is ring-shaped and sized to fit within a blood vessel in the body of the patient. An inner surface of the ring-shaped electrode forms an active measuring electrode surface. The blood flow measuring apparatus is suitable for use in a cardiac pacemaker.

TECHNICAL FIELD

[0001] The present invention relates to a blood flow measuring apparatus comprising a measuring electrode and a counter electrode intended to be brought into contact with the blood of a patient, and a measuring unit adapted to measure the voltage or current between said electrodes to determine therefrom the flow of blood.

BACKGROUND

[0002] There exist several methods of measuring the flow of body fluids. Thus the flow can be measured by Doppler sound probes, dilution methods where the flow is determined from probe temperature changes, cf. e.g. U.S. Pat. No. 5,598,847.

[0003] In U.S. Pat. No. 5,602,342 a method and a device are proposed for determining the flow of an electrolytic fluid from the measured voltage or current between two stationed electrodes emerged in the electrolytic fluid. No specific design of the measuring electrodes is shown in this document, but only schematically shown electrodes are positioned in the fluid path.

[0004] The purpose of the present invention is to provide a blood flow measuring apparatus based on the measuring principle described in the above U.S. Pat. No. 5,602,342 which does not disturb the flow and is well suited for continuous flow measurements.

DISCLOSURE OF THE INVENTION

[0005] This purpose is obtained by a flow measuring apparatus of the type described in the introductory portion of the description and having the characterising features of claim 1.

[0006] Thus in the apparatus according to the invention the measuring electrode surrounds the total flow in the body vessel in question. The measuring electrode is not positioned in the flow path as in the previous known flow measuring apparatus of this kind, thus avoiding disturbances of the flow. The measuring electrode of the apparatus according to the invention can be designed for location in any part of the human vascular system. Further, when using a point shaped electrode the measure result is depending on the position of the electrode in the flow inside the vessel in question. This is avoided with the present invention by using a ring-shaped measuring electrode with a size adapted to the dimensions of the vessel. The counter electrode can be any type of electrode, also an electrode intended for other purposes, e.g. a pacemaker stimulation electrode. The only requirement on the counter electrode is that no other current than the measuring current must pass this counter electrode during the measuring period. The blood flow measuring apparatus according to the invention is especially well adapted for measuring changes in the flow from a certain normal or reference flow.

[0007] To facilitate transveneous movement of the ring-shaped electrode to its final position the ring-shaped electrode is preferably designed to enable reduction of its diameter for the implantation procedure. One example of a technique for reducing the diameter for this purpose is called stent. According to advantages embodiments of the apparatus according to the present invention the ring-shaped electrode is foldable to enable reduction of its diameter for implantation. The ring-shaped electrode can be made up of several mutually foldable segments. Alternatively the ring-shaped electrode can be slotted to be wound together to reduce in this way the cross-section area for implantation.

[0008] To facilitate implantation the ring-shaped electrode is, according to another advantages embodiment of the apparatus according to the invention, designed for introduction, in is folded or wound together state, into a catheter suitable for insertion into a blood vessel, and further design to be pressed out of the catheter by a stiff wire to then relax to its ring-shape.

[0009] According to still other advantages embodiments of the apparatus according to the invention the ends of the ring-shaped electrode are flexible, preferably by forming a plurality of axial slits in the ends of the ring-shaped electrode or by providing the ends of the ring-shaped electrode by a wave-shape, the curvature of he wave-shape being small compared to the curvature of the ring. In this way the ends of the tube become soft so there will be no harmful cutting effect against the all of the vessel from the ring-shaped electrode.

[0010] According to yet other advantages embodiments of the apparatus according to the invention, wherein a lead is connecting the ring-shaped electrode to the measuring unit, an elongated part of plastic material is attached to the ring-shaped electrode to extend along the lead to serve as a support therefor. The lead is preferably passing through holes in the elongated part. This elongated part will serve as a mechanical relief to the lead. Because of the continuous moments of the implant every detail in the design must have an extreme fatigue durability and for this reason such a mechanical relief is of great importance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] To explain the invention in greater details embodiments of the invention chosen as examples, will now be described with references to the drawings, on which

[0012]FIG. 1 shows schematically a pacemaker system provided with a blood flow measuring apparatus according to the invention,

[0013]FIG. 2 shows a foldable ring-shaped electrode in its relaxed and folded state,

[0014]FIG. 3 shows a ring-shaped electrode provided with a longitudinal slit in a relaxed and winded together state,

[0015]FIG. 4 illustrates schematically a catheter suitable for use for insertion into a body vessel of the ring-shaped electrode,

[0016]FIGS. 5 and 6 show an alternative embodiment of the ring-shaped electrode of the apparatus according to the invention with a ring electrode slotted and flatted out, FIG. 5 showing a view from inside the ring electrode and FIG. 6 showing a lateral section view of the ring in its flatted out shape.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0017]FIG. 1 illustrates schematically a pacemaker system with leads 2 implanted into a heart 4. The leads 2 are connected to a pulse-generator and inside the pulse-generator housing 6 electronic circuitry of a flow measuring apparatus according to the invention is located. One of the leads 2 is connected to a ring-shaped measuring electrode 8 located in the coronary sinus vein 10. The other lead 2 is connected to an electrode 12 which can be used as a counter electrode in the measuring apparatus according to the invention but also be used for other purposes, e.g. supply of stimulation pulses from the pacemaker pulse generator to the heart 4. This counter electrode 12 must be in electrolytic contact with the measuring electrode.

[0018] In FIG. 2a a ring-shaped electrode is shown in its expanded implanted state. The electrode 14 is made foldable and in FIG. 2b the electrode 14 is shown folded together to reduce its diameter for facilitating implantation from outside the patient's body through the vascular system.

[0019]FIG. 3 shows an alternative embodiment of the ring-shaped measuring electrode 16 with a longitudinal slit 18 formed in the wall of the ring 16. In FIG. 3a the expanded state of the ring-shaped electrode 16 is shown and in FIG. 3b the slitted ring electrode is shown in a winded together state for reduction of its diameter to facilitate implantation.

[0020] Also other designs of the ring-shaped electrode are possible for making reduction of its diameter possible for the implantation. Thus the ring-shaped electrode can include a plurality of mutually foldable segments. The ring-shaped electrode can also be designed according to the so-called stent technique. The ring-shaped electrode than consists of a net or of a winded coil. During the implantation procedure the coil is comparably long and has a small diameter. The angle of the threads is low compared to the length during implantation. When the coil has reached its final position the coil is widened and shortened by drawing or pressing the two ends of the ring-shaped coil together or by using an inserted balloon. In this way the stent will fit to the inner side of the vessel.

[0021] As an alternative a tube-like catheter can be used for the implantation of the ring-shaped electrode into the vascular system. Such a catheter is illustrated in FIG. 4. The ring-shaped electrode is than folded together and inserted at the distal 20 of the catheter 22. The catheter 22 has a diameter and a length suitable for implantation from outside the patient's body through the vascular system. A stiff wire 26 is located in the catheter, and when the catheter 22 and the electrode 24 reach the desired position, which is preferably checked by X-ray, the stiff wire 26 is used to press the folded electrode 24 out of the catheter. When the electrode 24 is pressed out of the catheter 22 it will relax and assume Us ring-shape, adapted to the size of the vessel in question. The catheter 22 is also provided with a longitudinal slit so the lead 28 can easily be taken out of the catheter 22 to allow the catheter 22 to be moved away from the lead 28.

[0022] To avoid cutting against the wall of the body vessels it is desirable to make the ends of the ring-shaped electrode soft. This can be provided by a plurality of axial slits formed in the ends of the ring-shaped electrode or by a wave-shape as illustrated in FIG. 5. Thus FIG. 5 shows a view from the inside of a ring-shaped electrode 30 slotted and flatted out. The ring exhibit a wave-shape with a curvature that is small compared to the curvature of the ring itself. In the figure the waves are shown in an enlarged scale. The wave-shape can designed such that some of the waves also tend to fold out to improve the fixation of the ring against the wall of the vessel.

[0023] The electrode 30 is connected by a lead 32 to its associated electronics located in e.g. a pacemaker pulse-generator housing as described above. The ring electrode 30, which is preferably made of a biological stable plastics covered on its interior surface by a metallic layer 40, metallic net or the like, is designed with a prolonged supporting part 34 extending from the plastics of the ring 30. In the supporting part 34 a plurality of holes 36 is made through which the lead 32 is passing, cf. also FIG. 6. Such a mechanical relief for the lead 32 is very important.

[0024] Because of the continuous movements of every detail of this part of the apparatus is according to the invention an extremely high fatigue durability is required.

[0025] Connections between different metal parts in this construction is preferably realised by laser welding. At 38 in FIGS. 5 and 6 the lead 32 is contacted to the active metallic electrode surface 40 by welding. However, also other techniques like folding and pressing together different metal parts are possible. It is important that every connection between metal parts of the construction which may come into contact with body fluids must be protected against corrosion and measures must be taken to avoid influence on the measured values. Such a protection can be realised by applying a suitable plastic material, like e.g. spraying Teflon, evaporating Parylene or moulding silicon rubber. 

1. A blood flow measuring apparatus comprising a measuring electrode (8, 14, 16, 24, 30) and a counter electrode (12) intended to be brought into contact with the blood of a patient, and a measuring unit adapted to measure the voltage or current between said electrodes to determine therefrom the flow of blood, characterized in that said measuring electrode (8, 14, 16, 24, 30) is ring-shaped and sized to fit within a blood vessel (10) within the patient's body, and in that an inner surface (40) of said ring-shaped electrode is adapted to form an active measuring electrode surface.
 2. The apparatus according to claim 1, characterized in that said measuring unit is adapted to measure the current to the ring-shaped measuring electrode (8, 14, 16, 24, 30) while maintaining a constant voltage between said measuring and counter electrodes (8, 14, 16, 24, 30; 12).
 3. The apparatus according to claim 1 or 2, characterized in that the active surfaces of the measuring electrode (8, 14, 16, 24, 30) and the counter electrode (12) are of the same order of magnitude.
 4. The apparatus according to any one of the preceding claims, characterized in that said ring-shaped electrode (8, 14, 16, 24, 30) is sized to surround substantially the total flow in said vessel.
 5. The apparatus according to any one of the preceding claims, characterized in that said ring-shaped electrode (8,14, 16, 24, 30) comprises plastic material with said inner surface at least partly coated by an electrically conducting electrode material (40).
 6. The apparatus according to claim 5, characterized in that a platinum foil is attached to said inner surface of the ring (8,14, 16, 24, 30).
 7. The apparatus according to any one of the preceding claims, characterized in that the outer surface of said ring-shaped measuring electrode (8,14, 16, 24, 30) is insulated.
 8. The apparatus according to any one of the claims 1-6, characterized in that a second electrode is provided on the outer surface of the plastic ring.
 9. The apparatus according to any one of the preceding claims, characterized in that said ring-shaped measuring electrode includes a metal net.
 10. The apparatus according to any one of the preceding claims, characterized in that said ring-shaped electrode (14) is foldable to enable reduction of its diameter for implantation.
 11. The apparatus according to claim 10, characterized in that said ring-shaped electrode is made up of several mutually foldable segments.
 12. The apparatus according to any one of the claims 1-9, characterized in that said ring-shaped electrode (16) is slotted to be wound together to reduce the cross-section area for implantation.
 13. The apparatus according to any one of the claims 10-12, characterized in that said ring-shaped electrode (24) is designed for introduction, in its folded or wound together state, into a catheter (22) suitable for insertion into a blood vessel, and further designed to be pressed out of the catheter by a stiff wire (26) to then relax to its ring-shape.
 14. The apparatus according to any one of the preceding claims, characterized in that the ends of said ring-shaped electrode (30) are flexible.
 15. The apparatus according to any one of the preceding claims, characterized in that a plurality of axial slits are formed in said ends of said ring-shaped electrode.
 16. The apparatus according to any one of the claims 1-14, characterized in that said ends of the ring-shaped electrode (30) are wave-shaped, the curvature of the wave-shape being small compared to the curvature of the ring.
 17. The apparatus according to any one of the preceding claims, a lead (32) connecting said ring-shaped electrode (30) to said measuring unit, characterized in that an elongated part (34) of plastic material is attached to said ring-shaped electrode (30) to extend along said lead (32) to serve as a support therefor.
 18. The apparatus according to claim 17, characterized in that the lead (32) is passing through holes (36) in said elongated part.
 19. A pacemaker, characterized by an apparatus according to any one of the preceding claims. 